The present invention relates to a method for the heat treatment of a powdery material and apparatus therefor, and more particularly to a method for the heat treatment of a powdery material, which method is used for fusing in an airflow to round such particles containing a thermoplastic resin as toner particles for use as, for example, electrophotographic developer, and to apparatus used therefor.
As the method for rounding each of the particles of a powdery material as toner particles, there are known such a wet method as the spray-dry process that a powdery material is dissolved or dispersed into water or an organic solvent to make a solution or dispersed suspension, which is then fine-grained by a two-phase flow nozzle or rotary disc to be sprayed and dried in a heated airflow, and a dry method that toner particles are dispersed into a heated airflow to round the particles.
However, the above wet method has such disadvantageous problems that because the solvent contained in the particles must be mostly evaporated by the time when the sprayed particles are collected, the wet method requires the use of a large drying chamber taking a large spacing for the installation thereof and, if the solvent is nonwater, requires as additional installation for the recovery of the solvent, having possibility of causing such a danger as fire or toxicity to the human body.
On the other hand, the above dry method is also disadvantageous in respect that when heat-treating toner particles whose particle size is in the order of from several microns to several tenths microns there occur the production of coarse-grained toner particles due to the cohesion by thermal fusion of the particles and the adherence of the particles to the spray nozzle in the particle-dispersed airflow and to the internal surface of the chamber wall, thus reducing the yield and productivity, inviting an uneven heat-treatment condition.
One of the causes of the occurrence of the above-mentioned coarse-grained toner particles due to the cohesion by thermal fusion thereof and the uneven heat-treatment condition is such that the toner particles are not uniformly dispersed in the dispersing airflow, and both heated airflow and dispersing airflow are not uniformly thermally dispersed and collidingly mixed.
For example, Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) No. 140358/1978 discloses a method for blowing a dispersing airflow that circles round in the direction counter to that of a circling pressured heated airflow. In this method, however, not only is it difficult to form a uniform hollow cone-shaped dispersing airflow but the flying high up of the particles by the turbulent air around the orifice of the nozzle cannot be avoided, thus resulting in the increase in the adherence of the particles to the internal surface of the ceiling wall of the heat-treatment chamber.
Further, both round-circling airflows colliding with each other running in opposite directions produce a vigorous air turbulence, causing the toner particles to cohere by thermal fusion thereby to be coarse-grained, thus significantly increasing the adherence thereof to the ceiling wall surface, inviting the deterioration of the yield and productivity, so that the method still remains insufficient to solve such conventional disadvantages.
Japanese Patent O.P.I. Publication No. 60379/1978 discloses a method which is such that for the purpose of preventing the adherence by thermal fusion of thermoplastic particles to the wall of the spray nozzle that sprays a thermoplastic particles-dispersing airflow, the spray nozzle is provided around the periphery thereof with a flow path through which a cooling medium such as water or cooling airflow, thereby cooling the foregoing spray nozzle portion. The method, however, is not advantageous, either, in respect that the temperature of the internal wall of the spray nozzle becomes lowered to below dew point to condense waterdrops on the internal wall surface of the nozzle, causing the adherence of the thermoplastic particles, thus hindering a stable treatment of a large quantity of the particles.
And Japanese Patent Examined Publication No. 2165/1980 describes the conduction of cooling air into between a dispersing airflow and heated airflow for the purpose of preventing the adherence by thermal fusion of particles onto the orifice of the dispersing airflow spray nozzle. In this method, however, the temperature around the heat-encountering zone where the dispersing air and heated air are mixed becomes lowered by the conducted cooling airflow, thus deteriorating significantly the thermal efficiency, and further the flying up of the particles due to the turbulent air at the heat encountering zone cause by the cooling air conduction can not be avoided, so that it is impossible to completely prevent the adherence or cohesion by thermal fusion of the thermoplastic particles to the orifice and internal wall surface of the spray nozzle.
In referring further to the adherence of such particles to the internal wall surface of the chamber, generally speaking, powdery materials have such a nature that the more the particles thereof become finely grained and the lower the ambient airflow rate, the more does the adherence of the particles become increased. And because when adhering to a high temperature wall, the particles become fused, they tend to cause further adherence of other particles, thus producing finally aggregated masses of the fused particles. In the case of toner particles, the melting point and glass transition point thereof, although they depend on the resin used, are about 140.degree. C. and about 60.degree. C., respectively, so that in order to avoid the adherence or cohesion by thermal fusion of the toner that has attached to the heat-treatment chamber wall, the temperature of the wall needs to be lowered to not more than 60.degree. C.
Those heretofore used apparatus for rounding powdery particles of the kind adopt a method of either forcibly moving the particles by cooling means provided at the lower part of the apparatus or of conducting a cooled airflow into the midway of the recovery path for the heat-treated particles. These methods, however, are not designed so as to provide the prevention of the particles from attaching to the internal wall surface of the heat-treatment chamber and the cooling of the wall surface, so that they cannot avoid the adherence by thermal fusion of the powdery material or the occurrence of aggregated masses of the powdery material particles, so that if fragments of such attaching matter or masses come into the manufacturing product of toner during the course of the heat-treatment operation or during the time of cleaning after the operation, the product becomes defective, and removal of the defective lots leads to a large deterioration of the yield. If an attempt is made to reduce as much as possible the adherence by thermal fusion of or the attaching of toner particles, any high productivity of the toner cannot be carried out, and besides, the chamber needs to be of a much larger size.